1. Field of the Invention
The present invention relates to a pneumatic cylinder and, more particularly, to such a pneumatic cylinder which can prevent shocks that could damage its components by reducing cushioning pressure, and can provide speedy working stroke by reducing cushioning time.
2. Description of Related Art
Generally, the pneumatic cylinders convert fluid flowing under pressure to a linear motion to perform mechanical work. One example of a conventional pneumatic cylinder is illustrated in FIGS. 1 and 2. The conventional pneumatic cylinder includes a cylinder body defined by a barrel-like tube 102 and a pair of caps 104 and 106 fixedly disposed on opposite ends of the tube 102, respectively.
The pneumatic cylinder further includes a piston rod 110 slidably extending through the cap 106 into the inside of the tube 102 and a piston 108 fixed at the front end of the rod 110 which is located in the tube 102. Each of the caps 104 and 106 is provided with a plurality of fluid flow ports A and A' through which fluid can come in and leave from the inside of the tube.
The cylinder further includes a cushioning device for preventing shocks caused by reciprocating strokes of the piston 108. The cushioning device has a cushioning plunger 112 formed on the piston 108 and extending in a direction opposite to the rod 110, and a cushioning ring 114 fitted around an extension 113 connecting the piston 108 and the rod 110 with each other.
Further, the inner surface of the cap 104 is provided with a passage 116 which communicates with the port A and into which the plunger 112 can be slidably inserted. The inner surface of the cap 106 is also provided with a passage 118 which communicates with the port A' and into which the cushioning ring 114 can be slidably inserted. Accordingly, fluid within the cushioning chambers R and R' are to be returned to a fluid tank (not shown) through each passage 116 and 118 in accordance with the movement of the piston 108.
When the passage 116(118) is closed by fitting the cushioning plunger 112(the cushioning ring 114) thereinto in accordance with the movement of the piston, the fluid within the cushioning chamber R(R') is not returned to the fluid tank any more through the passage 116(118).
Accordingly, to provide a return passage for residual fluid, the caps 104 and 106 are respectively provided with orifices O and O' on their inner surfaces, which communicate with the ports A and A', respectively.
Further, cushioning valves 120 and 122 are respectively provided on the caps 104 and 106 to restrict the fluid amount passing through the respective orifices O and O' by regulating the opening thereof. This makes the cushioning of the piston which depends on the returning velocity of the fluid be regulated.
In the cushioning device as described above, when the pressurized fluid comes into the cushioning chamber R defined on the left side of the piston 108 through the port and passage A and 116 of the cap 104, the piston 108 within tube 107 forces to the right to perform the linear motion of the piston rod 110.
At this point, before the cushioning ring 114 formed on the extension of the piston 108 is inserted into the passage 118 of the cap 106 to close the passage 118, the piston rapidly moves to the right as the fluid within the cushioning chamber C' defined on the right side of the piston 108 returns to the fluid tank through the port and passage A' and 118.
However, once the cushioning ring 114 closes the passage 118, the piston slowly moves to the right as the residual fluid within the chamber C' returns to the fluid tank through the port A' via the orifice O'. That is, by regulating elastic force and returning velocity of the fluid, cushioning force applied to the piston 108 can be also regulated.
On the other hand, when the pressurized fluid comes into the cushioning chamber R' defined on the right side of the piston 108 through the port A' via the passage 118 of the cap 104, the piston 108 within tube 107 forces again to the left to accomplish the reciprocating motion of the piston rod 110.
At this point, in a similar manner, before the cushioning plunger 112 formed integrally on the left face of the piston 108 is inserted into the passage 116 of the cap 106 to close the passage 116, the piston rapidly moves to the left as the fluid within the cushioning chamber C defined on the left side of the piston 108 returns to the fluid tank through the port A via the passage 116.
However, once the cushioning plunger 112 obstructs the passage 116, the piston 108 slowly moves to the right as the residual fluid within the chamber C' returns to the fluid tank through the passage A via the orifice O. That is, cushioning force which is of elastic force of fluid generated by regulating its return velocity is applied to the piston 108.
FIG. 11 is a graph for comparing a shock power change in response to cushioning operation time and cushioning force of the piston 108 between the cushioning device of the present invention and this conventional cushioning device, wherein the curve line X' shows that the piston 108 receives cushioning force in a section Tss and the maximum shock power Pps is 10 kg f/cm.sup.2.
On the one hand, the cushioning force applied to the piston 108 which reciprocates in a state of receiving the shock power as described above can be regulated by the cushioning valve which regulates opening the orifices O and O' for restricting fluid flow amount.
However, since the pneumatic cylinder as described above suddenly restricts the returning velocity of fluid, the shock applied to the cylinder increase such that the cylinder has a short life. Additionally, since the cushioning operation time is getting longer, the stroke time of the cylinder is retarded.